The Science Guys

July 2002

Who first discovered that atoms are made up of other particles and how was this found?

The structure of the atom has always been one of the most intriguing areas of physics research. The Greek philosopher Democritus (460-370B.C.) conjectured that all things were composed of small, indivisible bits called "atoms," rendered from the Greek atomos meaning "indivisible." Democritus’ atoms came in different sizes and could combine or separate.

This idea of tiny, indivisible bits of matter persisted until the 1800’s. John Dalton (1766-1844), a great chemist, really started the modern atomic hypothesis. His atom however was like a solid billiard ball. Later, J.J. Thompson (1856-1940), discoverer of the electron, proposed what we call the "plum pudding model" of the atom. In his model, the atom was a mix of equal numbers of positive and negative charges.

The major experimental indication of the atoms’ structure was Rutherford’s Scattering Experiment. Ernest Rutherford (1871-1937) had been involved in studying the scattering of alpha particles by sheets of metal and mica. Some radioactive substances emit alpha particles (very massive, positively charged particles) and Rutherford thought that by studying how positive alpha particles in a beam were scattered by the atoms of a material one could infer the atom’s internal structure.

Hans Geiger, Rutherford’s colleague, suggested that their student Ernest Marsden look for deflections of alpha particles through very thin foils, feeling certain that large deflections would not occur. Indeed most alpha particles were scarcely deflected. But to Rutherford’s amazement, Marsden found that some alpha particles came straight back after being fired at a thin gold foil.

In Rutherford’s words, "It was the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you." The only way alpha particles could bounce directly back toward the "gun" that fired them was if the atom had a compact, positively charged core. An occasional alpha particle speeding directly toward such a core would be slowed, stopped, and rebound directly back due to electromagnetic forces. Rutherford knew from Thompson’s work that atoms contained electrons. Now however, he had found the nucleus of the atom, thus proving the atom had components.

We know today that the atom consists of a tiny (10-14 m) nucleus containing positive charges (protons) and neutral particles (neutrons), both of which consist of more fundamental particles called quarks. Negatively-charged electrons orbit the nucleus in cloud-like volumes of space designated "orbitals" in quantum theory. Since these electron clouds (10 -10 m) are 10 000 times larger than the radius of the nucleus, the atom is mostly empty space. A more detailed discussion of the atom will have to wait for more questions from eager Inquiring Minds readers! Be sure to send us your physics questions.